In most power semiconductor devices using power semiconductor elements, one side of the semiconductor element is in contact with a thin conductive-plate surface electrode formed on a surface of a thin insulator and the thin insulator is brought into contact with a metal heatsink, to ensure a cooling path for the semiconductor elements. Because of the fabrication process of semiconductor elements, it is common that the surface electrode is brought into contact with the collector side for cases with IGBTs, the drain side for cases with MOSFETs, and the cathode side for cases with diodes. For that reason, in a case of a power semiconductor device having a semi-bridge circuit configured with the upper-arm semiconductor element and the lower-arm semiconductor element connected in series, the positive polarity side of the semi-bridge circuit is brought into contact with the surface electrode. This creates a large stray capacitance between the positive polarity side and the heatsink. Since the negative polarity side of the semi-bridge circuit is, on the other hand, located away from the heatsink and in no contact with the surface electrode, a stray capacitance created between the negative polarity side and the heatsink is minute compared to that between the positive polarity side and the heatsink. Accordingly, the stray capacitance imbalance between the positive polarity-side and the negative polarity-side semiconductor elements and the heatsink increases noise, thus frequently causing an unwanted emission.
In order to deal with such a problem, for example, Patent Document 1 discloses a structure for in which both sides of each semiconductor element are covered with heatsinks and insulator plates are sandwiched between the heatsinks and metal patterns for the positive polarity-side and the negative polarity-side circuits to form capacitors, whereby the stray capacitances thereof are balanced to suppress noise.
In addition, there is disclosed another structure, in which an electrode electrically connected to heatsinks is disposed between wiring plates extending from the positive polarity-side terminal and the negative polarity-side terminal, to form capacitors between the electrode electrically connected to the heatsink and the positive and the negative polarity-side terminals, whereby the stray capacitances of the capacitors are balanced to suppress noise (refer to, for example, Patent Document 2).